Association of plasma 25-hydroxyvitamin D with indices of glucose tolerance and metabolic syndrome in middle-aged and older adults
Changes in body composition that occur with advancing age (i.e. increased fat mass and decreased lean mass) are associated with insulin resistance and elevated risks for type 2 diabetes mellitus (T2DM) and physical disability. Resistance training (RT) is recommended for older adults to combat these age-related changes. A growing body of evidence shows a higher vitamin D status, determined by plasma 25-hydroxyvitamin D (25(OH)D) concentration, to be associated with better insulin sensitivity and physical performance than a low vitamin D status. However, it is not known how vitamin D status may impact the benefits achieved from RT. For our first investigation, we evaluated the influence of baseline 25(OH)D concentrations on changes in body composition, muscle strength, and glucose tolerance in nondiabetic, weight-stable, older adults (n=35, aged 61±8 y, body mass index (BMI) 26.3±3.6 kg/m2; mean±SD) that performed RT 3x/wk for 12 wk. Based on the results of a 2-h oral glucose tolerance test performed at baseline and post-intervention, vitamin Dinsufficient subjects (<20 ng>/mL, n=7) had higher glucose area under the curve (AUC) and 2-h glucose concentration than vitamin D-sufficient subjects (≥20 ng/mL, n=28) at both time points (group main effects; glucose AUC, P=0.02; 2-h glucose, P=0.03) even after adjustment for age and BMI. Resistance training decreased fat mass (-6±7%, P<0.001) and increased lean body mass (2±3%, P<0.001), whole-body muscle strength (32±17%, P<0.001), and glucose tolerance (-10±16% in glucose AUC and -21±40% in 2-h glucose, P=0.001). Training-induced responses were not influenced by vitamin D status. Our findings suggest vitamin D insufficiency is associated with lower glucose tolerance in older, nondiabetic adults with RT-induced improvements in glucose control unable to negate the negative impact of an insufficient vitamin D status. In addition to aging, obesity is also a risk factor for T2DM and is associated with a higher prevalence of the metabolic syndrome (MetS). MetS is a clustering of metabolic abnormalities that increases the risk for T2DM and cardiovascular disease. Insulin resistance contributes largely to the development of the metabolic abnormalities that comprise MetS; therefore, the relationship between vitamin D status and MetS components may be mediated by the higher insulin sensitivity observed with higher 25(OH)D concentration. In addition, epidemiological evidence shows the prevalence of MetS and its components to be higher with lower 25(OH)D suggesting the well-documented seasonal variations in 25(OH)D concentration may be reflected in seasonal variations in the presence of MetS and its components. The second investigation was a cross-sectional study conducted to assess the association of plasma 25(OH)D concentration with indices of glucose tolerance and MetS and compare the prevalence of MetS and its components between seasons in 300 overweight and obese, middle-aged, Caucasian adults (BMI, 30.3±2.7 kg/m2; age, 48±8 y) at high risk for MetS. While measurements for the individual components of MetS did not differ among vitamin D status groups (low, ≤20 ng/mL; medium, 21-29 ng/mL; high, ≥30 ng/mL), lower vitamin D status was associated with higher fasting insulin, insulin AUC, and homeostatic model of insulin resistance (HOMA-IR) and lower insulin sensitivity index (ISI) in males, but not females (group × sex adjusted for BMI, P=0.048, 0.04, 0.07, and 0.05, respectively). Males with a low vitamin D status had higher insulin AUC and lower ISI than males with a high vitamin D status after adjustment for BMI (P<0.05). Plasma 25(OH)D was lower in the winter/early spring than the summer/early fall (20.2±5.1 vs. 25.3±7.7 ng/mL, respectively; P<0.001) with no differences observed in the prevalence of MetS or its components between seasons. Our results indicate that sex-specific evaluations should be considered in future research aiming to define the relationship between vitamin D status and glucose tolerance. Collectively, our investigations suggest a low vitamin D status (≤20 ng/mL) is associated with lower glucose tolerance and insulin sensitivity in nondiabetic adults; therefore, these individuals may be at a higher risk for T2DM than adults with a higher vitamin D status (>20 ng/mL). In conclusion, the Institute of Medicine currently recommends maintaining a 25(OH)D concentration ≥20 ng/mL for good bone health with our results suggesting this threshold concentration may also be appropriate for glucose tolerance in middle-aged and older, nondiabetic adults.
Campbell, Purdue University.
Off-Campus Purdue Users:
To access this dissertation, please log in to our